The pixels in plasma display panels and some other types of image display panels can only be driven at two levels, on or off. A so-called subfield method is therefore typically used in such display panels to achieve a display of motion picture with gray scale. This subfield method achieves a gray scale display by dividing each image field into a plurality of two-value subfields weighted for presentation on screen for different time periods. The weight of each subfield corresponds to the light emitted when that subfield is presented. More specifically, each subfield is assigned a luminance weight indicative of the number of times and the period for which pixels are switched on to display the subfield. A desired display luminance is achieved by selecting the combination of subfields which will achieve the desired gray scale.
FIG. 6 shows the time relationship the subfields of a single field in a typical subfield method. In this example, each field is divided into eight subfields, that is, subfields 1 to 8, which are assigned a luminance weight of 1, 2, 4, 8, 16, 32, 64, and 128, respectively. Each subfield is further divided into a set-up period T1, write period T2, and sustain period T3. The set-up period T1 discharges any residual charge in the subfield. Data for turning each pixel of the PDP either on or off is then written in the write period T2. Those pixels that are to be turned on based on the data written in the write period T2 are then turned on all at once during the sustain period T3, and the subfields are turned on in sequence from subfield 1 to subfield 8.
A 256-level display with gray scales from 0 to 255 can be achieved using subfields as shown in FIG. 6 by driving the subfields in various combinations. For example, a gray scale level of 7 can be achieved by turning pixels on for subfields 1 to 3, and a gray scale level of 21 can be achieved by using subfields 1, 3, and 5.
It is therefore possible with this subfield method to time-divide each image field into a plurality of subfields, select from among this plurality of time-divided subfields the subfields needed to achieve a desired gray scale level, and drive the display pixels for the time determined by the selected subfields to present the desired gray scale level.
In display devices using this subfield method are known, however, to suffer from pseudo contours appearing in the motion pictures. These pseudo contours will be further described below.
Let us assume that an image field has been time divided into subfields with weights of 1, 2, 4, 8, 16, 32, 64, and 128, and that image pattern X shown in FIG. 7 moves by two pixels horizontally on PDP screen 33. In addition, image pattern X comprises pixels P1 and P2 with gray scale level of 127, and adjacent pixels P3 and P4 with level of 128. The subfields that are driven to achieve these gray scale levels in image pattern X are shown in FIG. 8. Note that the horizontal direction in FIG. 8 corresponds to the horizontal direction of the PDP screen 33, and time is shown on the vertical direction. The emitting subfields are shaded.
When image pattern X is still, the gray scale level observed by a viewer is determined by the combination of emitting subfields through line A–A′, and the image gray scale level is normally perceived as intended. However, when the image pattern X moves horizontally across the screen as indicated in FIG. 7, the viewer's sight line would effectively moves in B–B′ or C–C′ direction in FIG. 8. When the sight line moves in B–B′ direction, the observer sees subfields 1 to 5 of pixel P4, subfields 6 and 7 of pixel P3, subfield 8 of pixel P2. Because these subfields are integrated in time field, the viewer would observe gray scale level 0. Conversely, when the sight line is through C–C′, the viewer observes subfields 1 to 5 of pixel P1, subfields 6 and 7 of pixel P2, and subfield 8 of pixel P3. In this case, the viewer would observe gray scale level of 255. More particularly, the perceived gray scale level is significantly different from the intended gray scale level of 127 or 128, and is seen by the human eye as a pseudo contour.
This problem of pseudo contours is particularly pronounced when this method of using weighted subfields is used and the luminance levels of adjacent pixels are 63 and 64, 191 and 192, and similar combinations which require a significant change in the pattern of emitting subfields to achieve a minimal change in gray scale. Contour lines such as these appearing only in moving picture images are known as pseudo contour noise and are a factor in image quality deterioration (see pseudo contour noise appearing in displays of PWM controlled moving pictures, Technical Report of the Inst. of Television Engineers of Japan, Vol. 19, No. 2, IDY95-21, pp. 61–66.).